The invention relates to a method and arrangement of facsimile picture transmission wherein digital signals, produced in a transmitter during the line-by-line scanning of pictures, are transmitted in coded form for redundancy-reduction to a receiver where they are decoded and printed by means of a printer. At the end of the signals associated with a scanned picture line an end-of-line control character is formed in the transmitter and transmitted to the receiver where the plurality of picture elements received after each end-of-line control character are counted and compared on the receipt of the next end-of-line control character with a predetermined number. In the event wherein the number of characters counted does not agree with the predetermined number, corrected signals for this picture line are printed instead of the incorrectly received signals of the picture line.
Such a procedure is disclosed in German Offenlegungsschrift No. 2,347,835. A frequently used form of coding is the so-called run-length coding, in which successive elements having the same luminance are counted and code words, corresponding to the number of elements, are transmitted. The code words generally have different lengths and are transmitted in immediate succession, that is to say without the insertion of separation marks. A disturbance during the transmission, even if it changes only one or a few bits of a code word of a picture line, disturbs the synchronization on separation of these code words in the receiver such that the entire remaining part of the picture line may be disturbed.
To obtain a minimum degree of synchronization, an end-of-line control character is transmitted at the end of each line, so that the picture line received thereafter can again be decoded correctly in the receiver. To prevent the disturbedly received picture line from being cancelled entirely during printing, which would result in a considerable shift of the printed picture in the case of frequently occurring disturbances and serious distortions in the transmission of pictures such as maps, the last correctly received picture line is printed instead of the disturbed line. However, it is also possible that a disturbance in the transmission disturbs an end-of-line control character itself so that it is no longer recognized as being correct in the receiver, or the disturbance of a code word with picture content may produce an end-of-line control character erroneously. With the prior art method, lines which should be printed would not be supplied at all or, erroneously, more than once in these cases.